1. Field of the Invention
The invention relates generally to the field of integrated circuit (IC) device packaging technology, and more particularly to substrate stiffening and heat spreading techniques in ball grid array (BGA) packages.
2. Related Art
Integrated circuit (IC) dies are typically mounted in or on a package that is attached to a printed circuit board (PCB). One such type of IC die package is a ball grid array (BGA) package. BGA packages provide for smaller footprints than many other package solutions available today. A BGA package has an array of solder balls located on a bottom external surface of a package substrate. The solder balls are reflowed to attach the package to the PCB. The IC die is mounted to the package substrate. Wire bonds typically couple signals in the IC die to the substrate. The substrate has internal routing which electrically couples the IC die signals to the solder balls on the bottom substrate surface.
A number of BGA package substrate types exist, including ceramic, plastic, and tape (also known as “flex”). In some BGA package types, a heat spreader/stiffener may be attached to the substrate to provide heat sinking, and to supply planarity and rigidity to the package.
Die-up and die-down BGA package configurations exist. In die-up BGA packages, the IC die is mounted on a top surface of the substrate or heat spreader/stiffener, on a side opposite that of the solder balls. In die-down BGA packages, the IC die is mounted on a bottom surface of the substrate or stiffener, which is the same side as the solder balls.
The tape substrate used in flex BGA packages is typically polyimide, which has very low values of thermal conductivity. Consequently, the IC die is separated from the PCB by the tape substrate thermal barrier. The lack of direct thermal connection from IC die to PCB leads to relatively high resistance to heat transfer from IC die to printed circuit board (theta-jb).
Furthermore, conventional BGA packages are subject to high thermal stresses that result from the heat given off during operation of the mounted IC die. The thermal stresses are primarily imposed on the IC die due to a mismatch of the thermal expansion coefficient (CTE) between the semiconductor die and the stiffener/heat spreader. The thermal expansion coefficient (CTE) of copper typically used for a heat spreader in a tape BGA package is approximately 17.4×10−6/° C. For a silicon IC die, the CTE is approximately 2.64×10−6/° C. Because of the difference in CTE values, changes in temperature during the BGA package assembly process may lead to high levels of thermal stress. As the IC die size increases for a BGA package, higher stress levels will occur at the interface of the IC die and stiffener/heat spreader. Consequently, cracks often occur on large semiconductor IC dies during the portions of the assembly process following the attachment of the IC die to the stiffener/heat spreader.
Hence, what is needed are BGA packages with improved heat spreading capabilities. What is also needed is a reduction in BGA package thermal stress during the assembly process, to improve packaging yields.